16.2 Byte-Compilation Functions

You can byte-compile an individual function or macro definition with
the byte-compile function. You can compile a whole file with
byte-compile-file, or several files with
byte-recompile-directory or batch-byte-compile.

Sometimes, the byte compiler produces warning and/or error messages
(see Compiler Errors, for details). These messages are recorded
in a buffer called *Compile-Log*, which uses Compilation mode.
See Compilation Mode in The GNU Emacs Manual.

Be careful when writing macro calls in files that you intend to
byte-compile. Since macro calls are expanded when they are compiled,
the macros need to be loaded into Emacs or the byte compiler will not
do the right thing. The usual way to handle this is with
require forms which specify the files containing the needed
macro definitions (see Named Features). Normally, the
byte compiler does not evaluate the code that it is compiling, but it
handles require forms specially, by loading the specified
libraries. To avoid loading the macro definition files when someone
runs the compiled program, write eval-when-compile
around the require calls (see Eval During Compile). For
more details, See Compiling Macros.

Inline (defsubst) functions are less troublesome; if you
compile a call to such a function before its definition is known, the
call will still work right, it will just run slower.

Function: byte-compilesymbol

This function byte-compiles the function definition of symbol,
replacing the previous definition with the compiled one. The function
definition of symbol must be the actual code for the function;
byte-compile does not handle function indirection. The return
value is the byte-code function object which is the compiled
definition of symbol (see Byte-Code Objects).

If symbol’s definition is a byte-code function object,
byte-compile does nothing and returns nil. It does not
“compile the symbol’s definition again”, since the original
(non-compiled) code has already been replaced in the symbol’s function
cell by the byte-compiled code.

The argument to byte-compile can also be a lambda
expression. In that case, the function returns the corresponding
compiled code but does not store it anywhere.

Command: compile-defun&optional arg

This command reads the defun containing point, compiles it, and
evaluates the result. If you use this on a defun that is actually a
function definition, the effect is to install a compiled version of that
function.

compile-defun normally displays the result of evaluation in the
echo area, but if arg is non-nil, it inserts the result
in the current buffer after the form it compiled.

Command: byte-compile-filefilename &optional load

This function compiles a file of Lisp code named filename into a
file of byte-code. The output file’s name is made by changing the
‘.el’ suffix into ‘.elc’; if filename does not end in
‘.el’, it adds ‘.elc’ to the end of filename.

Compilation works by reading the input file one form at a time. If it
is a definition of a function or macro, the compiled function or macro
definition is written out. Other forms are batched together, then each
batch is compiled, and written so that its compiled code will be
executed when the file is read. All comments are discarded when the
input file is read.

This command returns t if there were no errors and nil
otherwise. When called interactively, it prompts for the file name.

If load is non-nil, this command loads the compiled file
after compiling it. Interactively, load is the prefix argument.

This command recompiles every ‘.el’ file in directory (or
its subdirectories) that needs recompilation. A file needs
recompilation if a ‘.elc’ file exists but is older than the
‘.el’ file.

When a ‘.el’ file has no corresponding ‘.elc’ file,
flag says what to do. If it is nil, this command ignores
these files. If flag is 0, it compiles them. If it is neither
nil nor 0, it asks the user whether to compile each such file,
and asks about each subdirectory as well.

Interactively, byte-recompile-directory prompts for
directory and flag is the prefix argument.

If force is non-nil, this command recompiles every
‘.el’ file that has a ‘.elc’ file.

The returned value is unpredictable.

Function: batch-byte-compile&optional noforce

This function runs byte-compile-file on files specified on the
command line. This function must be used only in a batch execution of
Emacs, as it kills Emacs on completion. An error in one file does not
prevent processing of subsequent files, but no output file will be
generated for it, and the Emacs process will terminate with a nonzero
status code.

If noforce is non-nil, this function does not recompile
files that have an up-to-date ‘.elc’ file.